1. Field of the Invention
The present invention relates to a nitride based semiconductor laser device fabricated using a transparent substrate having conductive properties composed of gallium nitride (GaN), silicon carbide (SiC), or the like and a method of fabricating the same.
In this case, a Group III-V nitride based semiconductor such as BN (boron nitride), GaN (gallium nitride), AlN (aluminum nitride), InN (indium nitride), or TlN (thallium nitride), or their mixed crystal is referred to as a nitride based semiconductor.
2. Description of the Background Art
FIG. 12 is a schematic perspective view showing the structure of a conventional GaN based semiconductor laser device constructed by forming a nitride based semiconductor layer on a transparent GaN substrate having conductive properties.
The semiconductor laser device shown in FIG. 12 is constructed by stacking an n-AlGaN cladding layer 52, an n-GaN optical guide layer 53, an MQW active layer 54, a P-GaN cap layer 55, a p-GaN optical guide layer 56, and a p-AlGaN cladding layer 57 in this order on a transparent GaN substrate 51 having conductive properties. A ridge portion 50 is formed on the p-AlGaN cladding layer 57. An n-AlGaN current blocking layer 58 is formed on a flat portion of the p-AlGaN cladding layer 57, and a p-GaN contact layer 59 is further formed on the n-AlGaN current blocking layer 58 and the p-AlGaN cladding layer 57 including the ridge portion 50.
An n electrode 60 is formed on the whole of an upper surface of the GaN substrate 51, and a p-electrode 61 is formed on a predetermined region of the p-GaN contact layer 59.
In such a semiconductor laser device, a dielectric film (not shown) composed of one layer or a plurality of layers is formed at both facets along its cavity length. The following are two purposes of forming such a dielectric film in the semiconductor laser device.
One of the purposes is to protect the facets along the cavity length of the semiconductor laser device. That is, the dielectric film is exerted as a facet protective film, thereby preventing each of the layers exposed at the facets along the cavity length from being oxidized.
The other purpose is to adjust the total number of layers composing the dielectric film and the thickness thereof to adjust the reflectance of the dielectric film at each of the facets along the cavity length of the semiconductor laser device. Consequently, desired device characteristics can be obtained in the semiconductor laser device.
For example, in the dielectric film formed at each of the facets along the cavity length, the number of layers composing the dielectric film and the thickness thereof are adjusted such that its reflectance at the facet on the side of laser light emission (hereinafter referred to as a front facet) is low and its reflectance at the facet on the opposite side thereof (hereinafter referred to as a rear facet) is high. Consequently, the amount of light emitted from the front facet can be increased, thereby increasing the output power of the semiconductor laser device.
As described in the foregoing, in the semiconductor laser device, dielectric films having different reflectances are respectively formed at the front facet and the rear facet. In a semiconductor laser apparatus using such a semiconductor laser device, the semiconductor laser device is arranged on a sub-mount or the like in correct forward and backward directions so as to be normally operated.
However, it is difficult to distinguish the forward and backward directions in the conventional semiconductor laser device, as shown in FIG. 12. Accordingly, it takes much time to distinguish the directions. In a semiconductor laser apparatus using such a semiconductor laser device, therefore, the yield in the assembling process is low, and the work efficiency in the assembling process is reduced. As a result, the fabrication efficiency of the semiconductor laser apparatus is reduced, and the fabrication cost thereof is raised.